The present invention relates to optical amplifiers utilized to amplify optical signals transmitted within optical communications systems. More particularly, the present invention relates to a hybrid optical amplifier comprising a distributed Raman amplifier optically coupled in series to an Erbium-Doped Fiber Amplifier.
Raman amplifiers are being increasingly utilized in tandem with Erbium-Doped Fiber Amplifiers (EDFAs) within optical communications systems. It is known that such a hybrid Raman/EDFA system can provide improved signal-to-noise characteristics over simple EDFA""s used alone. These improved signal-to-noise characteristics can permit either the utilization of increased span lengths between amplifiers or else the operation of receivers at reduced optical power levels. For instance, when utilized in a system comprising common SMF fiber, the use of a Raman amplifier in conjunction with an EDFA can improve receiver sensitivity by approximately seven dB.
A typical hybrid Raman/EDFA amplifier apparatus 100 is shown in FIG. 1. The backward-propagating light of at least one Raman pump laser 107 amplifies the input optical signal along the transmission fiber 106 with a very low effective noise figure, which can be zero or even negative. A Wavelength Division Multiplexer (WDM) coupler 102, which can, for instance, be based upon fiber-coupler, thin-film filter or other conventional WDM technology, is used to combine the optical signal and the Raman pump laser light. The Raman pump laser light is inserted into the optical system through the P-port of the WDM coupler 102 and is directed to the input fiber 106 from the C-Port of the WDM coupler 102. Concurrently, an optical signal is directed to the C-Port of the WDM coupler 102 through the transmission fiber 106. The backward propagating Raman pump laser light provides Raman amplification to the signal along the length of fiber 106 which acts as pre-amplification for the EDFA 104. The signal then exits the WDM coupler 102 from the R-Port, from which it is directed, through optical coupling 108, to an EDFA 104 for further amplification. The amplified signal is then output to an output fiber 110.
Typically, multiple pump lasers are employed in a conventional hybrid Raman/EDFA system. The lights from the various pump lasers may comprise different polarization, so as to compensate for polarization dependent Raman gain in the input fiber 106 or may comprise different pump wavelengths, so as to broaden and flatten the Raman gain in the input fiber 106. Although the example illustrated in FIG. 1 shows two Raman pump lasers 107, typically six to eight such pump lasers are utilized. The lights of the plurality of pump lasers 107 are combined by at least one pump combiner 109. The pump combiner 109 may comprise one or more simple optical couplers such as fused fiber couplers or beam splitters used in the reverse sense, and/or one or more polarization combiner elements and/or WDM filters.
FIG. 2 illustrates a conventional structure of a WDM 102 for combining the Raman pump laser light together with the signal. Nominally, all the Raman pump laser light is directed from the WDM coupler 102 into the transmission fiber 106 so as to propagate in a reverse direction to the optical signal. However, since the combined pump laser power from multiple pump lasers could be as high as 30 dBm, there can be considerable leaked Raman pump light at the R-port of the WDM coupler. Although some available WDM couplers can achieve optical isolation that is as high as 50 dB between the P-port and the R-port, the leak pump power may still be comparable to the signal power.
Usually, the wavelength of the leaked Raman pump laser is shorter than the signal band and, thus, will not be amplified by EDFA. Eventually, the leaked Raman pump laser light will be filtered out from subsequent optical components before it reaches a receiver, so a small amount of leaked Raman pump laser light is harmless for data transmission. Nonetheless, the leaked Raman pump laser light can cause problems for input power monitoring of EDFA 104 when the leaked pump power is comparable to the signal power. The input power monitoring of the EDFA is essential for the a variety of necessary functions such as automatic gain control, automatic channel number recognition, etc. Conventionally, the power of the leaked Raman pump laser light is reduced by inserting a pump wavelength filter 112 between the R-port and the EDFA 104. Unfortunately, the use of the filter 112 also caused undesirable reduction of the signal power due to the insertion loss of the filter, thereby negating some of the advantages of using Raman pre-amplification.
Accordingly, there exists a need for an improved apparatus and method for channel monitoring in a hybrid Raman/EDFA optical amplifier. The apparatus and method should address the above-mentioned problems without significant insertion loss penalty.
To address the above-mentioned problem related to leaked Raman pump laser light, the present invention provides a hybrid Raman/EDFA optical amplifier that utilizes a first optical detector that generates a first electrical or electronic signal that is proportional the combined optical power of the signal light and the leaked pump laser light, a second optical detector that generates a second electrical or electronic signal that is proportional to the power of the pump laser light and an electronic subtraction circuit that receives the first and second electrical or electronic signals and generates a difference signal that is proportional to only to the optical power of the signal light and insensitive to the power of the Raman pump laser light. The difference signal is utilized by an EDFA control circuit to control and/or adjust the operation of the EDFA based upon changes in the power of the input optical signal.
An exemplary embodiment of a hybrid Raman/EDFA optical amplifier apparatus in accordance with the present invention comprises: an Erbium-Doped Fiber Amplifier (EDFA), an output fiber optically coupled to an output of the EDFA, a Wavelength Division Multiplexer (WDM) coupler optically coupled to the input of the EDFA and outputting an optical signal to the EDFA, an input fiber optically coupled to the WDM for inputting the optical signal to the WDM and for receiving a Raman pump laser light from the WDM, a first optical tap optically coupled between the EDFA and the WDM, a first monitor detector optically coupled to the first optical tap, an electronic amplifier electrically coupled to the first monitor detector, at least one Raman pump laser optically coupled to the WDM emitting the Raman pump laser light, a second optical tap optically coupled between the Raman pump laser and the WDM, a second monitor detector optically coupled to the second optical tap, an electronic subtract circuit electrically coupled to the second monitor detector and the electronic amplifier and an EDFA control circuit electrically coupled to the electronic subtract circuit and to the EDFA.
The electronic subtract circuit receives an amplified form of the first electrical or electronic signal from the electronic amplifier, receives the second electrical or electronic signal from the second monitor detector and outputs a signal to the EDFA control circuit that is the difference between the two electrical or electronic signals. This difference signal is proportional only to the power of the optical signal. The EDFA control circuit adjusts the operation of the EDFA based upon the optical signal power determined from the difference signal.
The optical amplifier of the present invention is robust against pump laser power fluctuations and adjustments and provides input signal power or channel count monitoring without significant insertion loss penalty.